1. Field of the Invention
The present invention generally relates to digital communication systems, and more particularly to a system and method for communicating pre-connection information in a digital communication system.
2. Discussion of the Related Art
Since the first T-Carrier system was introduced into commercial service by AT&T in 1962, Integrated Digital Networks (IDN's) have continually evolved, primarily driven by the need to provide economic voice (and data) communications. One such IDN is called the Integrated Services Digital Network (ISDN), which combines the coverage of the telephone network with the data-carrying capacity of digital data networks. In this regard, the term integrated refers to the ability to simultaneously carry digitized voice and data on the same digital transmission link, and through the same digital exchanges.
Standards for ISDN have been defined by the International Telecommunication Union (ITU), Telecommunication Standardization Sector (ITU-T). The ITU-T (formerly CCITT) recommendation I.120 defines both the principals of ISDN, as well as the evolution of ISDNs. In this regard, the main feature of the ISDN concept is the support of a wide range of voice and nonvoice applications in the same network. A key element of the service integration for an ISDN is the provision of a range of services using a limited set of connection types and multipurpose user-network interface arrangements. ISDNs support a variety of applications, including both switched (circuit switched and packet switched) and nonswitched connections. New services introduced into an ISDN should be compatible with the 65 kilobits per second switched digital connections. An ISDN is to be structured in accordance with a layered protocol that can be mapped into the seven-layer Open Systems Interconnection (OSI) model. In regard to the evolution of ISDNs, the ITU-T recommendation I.120 provides that ISDNs will be based on the concepts developed for telephone IDNs and may evolve by progressively incorporating additional functions and network features, including those of any other dedicated networks, such as circuit switching and packet switching.
The further development of ISDN is governed by a set of recommendations issued by ITU-T, called the I Series Recommendations. First issued in 1984, a more complete set of these recommendations has now been issued, and is centered on three primary areas. These include the standardization of services offered to users, the standardization of user-network interfaces, and the standardization of ISDN capabilities.
The foregoing has been presented merely for purposes of introduction, since the background and evolution of ISDNs is well known and documented. Suffice it to say that ISDN provides a very robust communication network in regard to both speed and flexibility. In general, the transmission structure of an ISDN is defined by B and D channels. The B channel is a user channel that can be utilized to transmit data, encoded digital voice, or a mixture of the two. The B channel is defined by a transmission rate of 64 kilobits per second. Furthermore, the B channel can support three types of connections: circuit-switched, packet-switched, and semi-permanent. The circuit-switched connection is established with another network user, when a first user places a call. In a packet-switched connection, a user is connected to a packet switching node, and data is exchanged with one or more other users in accordance with the X.25 protocol standard. Semi-permanent connections (sometimes referred to as permanent virtual circuits) are connections set up between users in accordance with the prior arrangement, and do not require a call establishment protocol.
The D channel is used for signaling and control. In this regard, the D channel carries common channel signaling information to control circuit-switched calls on associated B channels. It may also be used for packet switching or low speed telemetry, if no signaling or control information is otherwise waiting for transmission.
Using these two types of channels (which are time division multiplexed), ISDN provides both a basic access, and a primary access. The basic access is defined by two full-duplex B channels and one full-duplex D channel. The control and signaling information carried on the D channel provides the control and signaling for both of the B channels. ISDN primary access is generally directed for systems with greater capacity requirements, such as local area networks, multi-branch exchanges, etc. The primary access ISDN is typically based upon either a 1.544 mega bit per second transmission structure (corresponding to the T1 transmission structure) or a 2.048 mega bit per second transmission structure (based upon the European standard). Typically, the channel structure for the primary access ISDN is 23B channels plus one 64 kilobit per second D channel (for the 1.544 mega bit per second structure), or 30B channels plus one 64 kilobit per second D channel (for the 2.048 mega bit per second structure).
As previously mentioned, ISDN is defined in accordance with the layered protocol of the OSI model. Layer one, or the physical layer, functions to encode digital data for transmission, ensure full-duplex transmission of B and D channel data, multiplexes the channels to form either basic or primary access transmissions, and other known functions. Layer two, or the data link layer, establishes a protocol that is needed for communication. The ITU-T has defined a data link control protocol for the D channel, known as Length Access Protocol D channel or LAPD. LAPD is defined by ITU-T recommendation Q.921. Operating above the LAPD, is layer 3, or the network layer. The ITU-T recommendation Q.931 defines a network-layer protocol, which makes use of the D channel and defines the procedures for establishing, maintaining, and clearing of network connections at the ISDN user-network interface.
In accordance with the foregoing standards and protocols, a variety of information is exchanged between a calling party and a called party in an ISDN, prior to the establishment of the connection between the two parties. In this regard, the Q.931 recommendation defines a number of call establishment messages that exchange information prior to the establishment of the connection. These messages include: ALERTING, CALL PROCEEDING, CONNECT, CONNECT ACKNOWLEDGE, PROGRESS, SETUP, AND SETUP ACKNOWLEDGE.
By way of example, the SETUP MESSAGE includes a variety of information elements. One such element is the calling party number and calling party subaddress. As is known, this information is placed into the set up message at the first local exchange (i.e., the local exchange nearest the calling party). The calling party number identifies the telephone number of the calling party. Therefore, upon receiving the SETUP message, the called party may evaluate the calling party number to determine the identity of the calling party. The subaddress information is used to identify address information internal to the calling party. For example, if the call originates from a private branch exchange (PBX) or a local area network, the subaddress information element may be used to identify a particular extension or terminal at the calling party premises.
It is appreciated that, based upon information such as the calling party number, a called party may choose to, for example, accept or decline the incoming call. In this regard, if the called party wishes to accept the incoming call, it will then issue a CONNECT message to return to the calling party.
A number of additional information elements that are exchanged in a pre-connect communications time period are well known, and defined within the ITU-T recommendations. While useful information may be communicated within the information elements presently provided by the defined ISDN standards, other information is desired. For example, a called party may often desire to know the reason that the connection is being requested by a calling party. Presently, no such information element is provided or defined by the ISDN specifications. Accordingly, there is a need to provide a system and method that can communicate additional calling information prior to the establishment of the call connection.